James Adam, The Republic of Plato

δύνηται has been suspected by Stephanus, Ast, and Herwerden. The word could well be spared, but, as it is in all the MSS, was read by Stobaeus (Flor. 9. 67) and is not in itself an error, we are hardly justified in resorting to excision. The subjunctive depends of course on ὅταν, and καί connects δύνηται with ἔχῃ. By reading τῶν ἐν αὐτῷ (not αὑτῷ) θρεμμάτων, and making τὸ τοῦ βελτίστου εἶδος the subject to δύνασθαι and θεραπεύειν, Schneider makes it rather easier to refer δύνηται to ὅταν τις: but on the whole it is more natural to retain αὑτῷ and make the subject of δύνασθαι the same as that of δύνηται, viz. the individual himself. Even at the cost of a little awkwardness, Plato prefers to make the sentiment expressed by καὶ τὰ θωπεύματα —μανθάνειν co-ordinate with, rather than a consequence of, ὅταν τις ἀσθενὲς κτλ. For the imagery of the sentence cf. (with J. and C.) VI 493 A ff.

ἵνα καὶ κτλ. is Plato's justification for assigning the farmers and artisans to a dependent position in his city. For the syntax of ὑπὸ ὁμοίου—οἵουπερ see III 402 A note